Electronic structure methods are important tools used to understand, study and predict the behavior of molecular systems. Advancement of these methods is the main objective of this proposal. The applicability of accurate electron correlation methods is limited by the steep increase in the computational cost when the size of the molecular system is increased.In this project a new approach is proposed where the computational cost of the electron correlation part becomes constant with the size of the molecular system. This is obtained using multi-level methods where different levels of theory can be applied to different parts of the system. In this way, the calculation of the total wave function is avoided and only the part relevant for a local molecular property is determined. The methods are said to have size-intensive complexity.The multi-level approach will be developed in many directions. For single molecules I will develop coupled cluster wave function and response methods, together with multi-configurational self-consistent field methods using density matrix renormalization group theory. The multi-level approach will also be developed for systems with periodic boundary conditions.The developed methods will be used to simulate chiroptical properties of molecules and study the dynamics of excited states for molecules with biological interest. The multi-level methods will also be used to simulate solvent effects on molecular properties.